submenus for editing fields should be deduced automatically, not hardcoded

At the moment, Admin Toolbar shows submenus for node types and their fields, and also for taxonomy. However, any entity types provided by contrib modules or custom code don't get this. That's quite a severe limitation.

The problem is that the submenus for the entities that have them come from hardcoded values:

  $content_entities = array(
    "block_content" => array(
      "content_entity" => "block_content",
      "config_entity" => "block_content_type"
    ),
    "comment" => array(
      "content_entity" => "comment",
      "config_entity" => "comment_type"
    ),
    "contact" => array(
      "content_entity" => "contact_message",
      "config_entity" => "contact_form"
    ),
    "node" => array("content_entity" => "node", "config_entity" => "node_type"),
    "taxonomy" => array(
      "content_entity" => "taxonomy_term",
      "config_entity" => "taxonomy_vocabulary"
    ),
  );
  // Add  common links to entities.
  // The common links for entities are :
  // - Editing entity, Deleting entity,manage fields for entity
  //, manage form display for an entity, entity view display and others links relating to entity
  foreach ($content_entities as $module_name => $entities) {
    $config_entity = $entities['config_entity'];
    $content_entity = $entities['content_entity'];

Instead of hardcoding these types, they should be obtained from entity info dynamically.